Synthesis of double-shelled periodic mesoporous organosilica nanospheres/MIL-88A-Fe composite and its elevated performance for Pb2+ removal in water

Herein, we report the synthesis of double-shelled periodic mesoporous organosilica nanospheres/MIL-88A-Fe (DSS/MIL-88A-Fe) composite through a hydrothermal method. To survey the structural and compositional features of the synthesized composite, a variety of spectroscopic and microscopic techniques, including FT-IR, XRD, BET, TEM, FE-SEM, EDX, and EDX-mapping, have been employed. A noteworthy point in this synthesis procedure is the integration of MOF with PMO to increase the adsorbent performance, such as higher specific surface area and more active sites. This combination leads to achieving a structure with an average size of 280 nm and 1.1 μm long attributed to DSS and MOF, respectively, microporous structure and relatively large specific surface area (312.87 m2/g). The as-prepared composite could be used as an effective adsorbent with a high adsorption capacity (250 mg/g) and quick adsorption time (30 min) for the removal of Pb2+ from water. Importantly, DSS/MIL-88A-Fe composite revealed acceptable recycling and stability, since the performance in Pb2+ removal from water remained above 70% even after 4 consecutive cycles.

www.nature.com/scientificreports/ were measured on a Quantachrome Instruments version 2.2 using N 2 as the adsorbate at − 196 °C. Transmission electron microscopy (TEM) was carried out using an EM10C-100 kV microscope (ZEISS Company). FE-SEM images, EDX, and EDX-mapping were recorded by TESCAN (model: Sigma VP) scanning electron microscope operating at a low accelerating voltage of 15.00 kV and resolution of about 500 nm (ZEISS Company). Inductively coupled plasma optical emission spectroscopy (ICP-OES) was accomplished with a Varian Vista Pro CCD (Australia). UV-Vis spectra of the samples were obtained on a Hitachi UV-2910 spectrophotometer.
Synthesis of double-shelled periodic mesoporous organosilica nanospheres. Double-shelled PMO nanospheres were obtained via a sol-gel process based on literature reports 140 . In a typical synthesis, 0.16 g of CTAB was combined with a mixed solution of ethanol (30 mL), concentrated ammonia (1.0 mL), and deionized water (75 mL) at 40 °C for half-hour. Afterward, a mixture containing BTSE (0.119 g, 0.33 mmol) and TEOS (0.116 g, 0.56 mmol) was quickly added to the above mixture under vigorous stirring (1100 rpm) at 40 °C and kept for 24 h. To obtain two-layered mesostructured organosilica spheres, a mixture of TEOS and BTSE with an initial molar ratio was added to the mixture of the previous step. After further stirring for 24 h at 40 °C, the slurry was collected by centrifugation and washed with ethanol. The periodic mesostructured organosilica spheres were re-dispersed in 360 mL of deionized water and then transferred to a Teflon-lined stainless-steel autoclave, which was heated in an airflow electric oven at 140 °C for 5 h. After cooling the autoclave to room temperature, the product was collected by centrifugation. Subsequently, by the solvent-extraction process containing a solution containing 180 mL of ethanol and 360 µL of concentrated HCl, CTAB templates were removed from the product. Finally, double-shelled ethane-bridged PMO nanospheres were obtained after washing with ethanol three times and drying under a high vacuum at 80 °C overnight. Fe composite has proceeded through hydrothermal treatment. Firstly, a certain amount of the double-shelled ethane-bridged PMO powder (10 wt%) was dispersed into 20 mL ultrapure water in a bath sonicator at room temperature for 1 h. Afterward, 1 mmol of FeCl 3 ·6H 2 O (0.27 g) and 1 mmol fumaric acid (0.116 g) were dissolved in a mixture of DMF and the ethanolic solution previously prepared with a proportion in volume 4.5:1 obtaining a 0.8:1 NaOH to Fe ratio in the reaction media. Then, double-shelled ethane-bridged PMO solution was gradually dropped into the above solution and sonicated for 15 min under the same conditions. Then the mixture was transferred into a 100 mL autoclave, sealed and heated to 65  Analytical methods. The adsorption capacity ( q e , mg g −1 ) of Pb 2+ by the solid adsorbent composite at equilibrium and as well as the removal efficiency (%) (R) of Pb 2+ was calculated by the following formula: where C 0 (mg/L) is the initial concentration of Pb (II), and C e (mg/L) is the equilibrium concentration in the liquid phase. V is the volume of solution (mL), and m is the amount of the adsorbent (mg).
The FTIR spectra of (a) DSS nanospheres, (b) MOF MIL-88(A)-Fe, and (c) DSS/MIL-88(A)-Fe composite, have been shown in Fig. 1. As it is evident from Fig. 1a, the typical absorption bands located at 1084, 819, and 465 cm −1 are assigned to the asymmetric, symmetric, and bending vibrations of the Si-O-Si bond, respectively. The FT-IR spectra of the DSS nanospheres showed absorbance bands of about 2930 cm −1 , which can be assigned to the vibration of C-H bond in -CH 2 -CH 2 -group, clearly indicating the ethane-bridged frameworks.
The broad absorption band at 3440 cm −1 and the distinctive band at 1623 cm −1 can be related to the stretching and bending modes of the surface-attached hydroxyl groups (ν O-H) and adsorbed water molecules in DSS nanospheres, respectively 140 . In the FTIR spectrum of MOF MIL-88(A)-Fe (Fig. 1b), the band corresponding to the ν(C=C) for the fumarate ligand emerges as a sharp band in the region of 1690 cm −1142 . Also, two influential bands at 1607, and 1398 cm −1 can be attributed to the asymmetric and symmetric vibration modes of the carboxyl group from fumaric acid, respectively. The characteristic peak at 790 cm −1 can be associated to the C-H bending vibration of the organic linker 143 . Besides, the absorption band at 640 cm −1 is allocated to carbonyl group 144,145 . In the case of DSS/MIL-88(A)-Fe composite, as can be observed in Fig. 1c, the characteristic stretching vibration of O-H (attributed to the double-shelled SiO 2 ) is found at 3420 cm −1 , which is covered by O-H vibrational mode of water content (related to the adsorption of moisture in the air to MOF. Furthermore, the definite structure of the nanocomposite was corroborated by the advent of the absorption bands at 1608 and 1398 cm −1 (attributed to coordination between the carboxyl group and Fe 3+ ), and absorption bands at 1054 and 800 cm −1 (associated    www.nature.com/scientificreports/ 2θ = 8.5° attributed to the (100) crystallographic facet developed more than that of pure MIL-88(A)-Fe (Fig. 2c). The shift might be due to the double-shelled periodic mesoporous organosilica nanospheres that controlled the MIL-88(A)-Fe MOF crystal orientation in the modified composite. The XRD results suggested that the pure MIL-88(A)-Fe MOF and desired composite were successfully synthesized. It is known that the BET surface area and pore structure of the as-synthesized nanocomposite are substantial factors, which influence the catalytic activity. To gain further insights into the textural properties (the pore-size distributions and BET surface areas), N 2 adsorption-desorption analysis of DSS nanospheres (a), MIL-88(A)-Fe MOF (b), DSS/MIL-88(A)-Fe nanocomposite (c), and DSS/MIL-88(A)-Fe nanocomposite after Pb 2+ adsorption (d) were presented in Fig. 3. As seen in Fig. 3a, DSS nanospheres exhibit typical type IV isotherm with a large hysteresis loop indicating the presence of mesoporous structure with a high surface area of about 244.87 m 2 g −1 and pore volume of 0.864 cm 3 g −1 . It was found in Fig. 3b that the hysteresis loops in the isotherm curve of MIL-88(A)-Fe MOF can be attributed to typical type IV isotherm with H3 hysteresis loop 148 . Furthermore, the derived BET (obtained from Brunauer-Emmett-Teller (BET) theory) surface area and pore volume were estimated to be 236 m 2 g −1 and 0.180 cm 3 g −1 , respectively. As it is evident from the data summarized in Table 1, the BET surface area of DSS/MIL-88(A)-Fe nanocomposite is larger than the pure DSS and MOF (Fig. 3c). The larger pore volume of DSS/MIL-88(A)-Fe nanocomposite compared to the pure DSS and MOF offered a mesoporous architecture for composite samples, providing a suitable pathway for mass transport (Table 1). Moreover, the mean pore volume of DSS/MIL-88(A)-Fe nanocomposite (2.919 nm) is slightly lower than those of the pure DSS (14.115 nm) and MOF (3.056 nm), which is probably due to the synergistic effect and implies the successful combination of MOF and PMOs. Furthermore, investigating on the Barrett-Joyner-Halenda (BJH) pore size distribution which was calculated using the adsorption branch (presented in Fig. 3e) clearly shows three peaks centered at 8, 19 and 61 nm, corresponding to the mesoporous of the shell, hollow void (the shell-in-shell distance) and also macro porous of MOF, respectively. By comparing the BJH before and after Pb 2+ adsorption, the conclusion could be derived that no significant changes were observed in the pore size distribution (Fig. 3e,f).
The morphological studies of DSS/MIL-88(A)-Fe nanocomposite was performed by investigating the field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The obtained results are presented in Fig. 4. In order to testify the well synthesized DSS/MIL-88(A)-Fe nanocomposite, initially we examined the pure DDS nanospheres and MIL-88(A)-Fe MOF by FE-SEM analysis. As can be observed from Fig. 4a, DDS exhibits a uniformly spherical shape, monodisperse size distribution, and high surface area with particle sizes of about 280 nm. Also, the original MIL-88(A)-Fe MOF shows well-crystallized rods with a hexagonal face and an average size of 1.2 μm (Fig. 4b). The formation of DSS/MIL-88(A)-Fe nanocomposite is obviously confirmed from the FE-SEM images (Fig. 4c,d). As revealed by Fig. 4c, For further insight on the morphology of the synthesized DSS/MIL-88(A)-Fe nanocomposite, the TEM images were surveyed and presented in Fig. 5a-c. The TEM images of the nanocomposite confirm the existence of bright and dark areas in the double-shelled structures. Furthermore, as it is evident from Fig. 5a,b, the cavities observed from TEM images suggested the presence of mesoporous channels on the surface of the DSS nanospheres. Of note that the formation of the hexagonal-rods structure of MIL-88(A)-Fe MOF was clearly observed in Fig. 5c. Also, the thickness of the external layer, internal layer and the hollow void (the shell-to-shell distance) was estimated to be ∼13, ∼ 20 and ∼33 nm, respectively. Accordingly, the overall outer diameter is about ∼280 nm for DSS nanospheres which is very close to the FE-SEM data.
Surveying the EDX spectrum confirmed the existence of C, O, and Fe elements in the nanocomposite structure (Fig. 6a). After adsorption, the Pb 2+ signals have been detected in the used adsorbent surface (Fig. 6b).
To further evaluate the DSS/MIL-88(A)-Fe nanocomposite, EDX-mapping analysis was performed and the presence of C, O, Si, and Fe with uniform distributions was approved (Fig. 7a). In addition, after Pb 2+ adsorption, the elemental distributions were almost identical. These findings evidenced that Pb 2+ had successfully attached to the adsorbent surface (Fig. 7b).    www.nature.com/scientificreports/ As can be seen from Fig. 10, with the increase of adsorbent dosage, the percentage removal initially increases, which could be due to the increase of the total surface area of the adsorbent, and the adsorption site number of the adsorbent in a certain amount of solution. But beyond a value of 20 mg, the percentage removal reaches an almost constant value. This may be due to an overlapping of adsorption sites and, consequently, of the adsorbent particles overcrowding. Maximum removal of 86.6% was observed at an adsorbent dosage of 20 mg/L (for economic purposes) at pH 6.
Effect of pH value. It is well-known that the pH value of the solution is an essential factor affecting the adsorption performance of Pb 2+ . For this purpose, different pH values (3, 4, 5, 6, and 7) affecting the adsorption performance of Pb 2+ by DSS/MIL-88A-Fe were investigated (Fig. 11). The adsorption capacity of Pb 2+ on composite depends on the pH value, which increased slowly with an increase of pH until pH 6.    Adsorption isotherm. To obtain mechanism information on the adsorption process, adsorption isotherm models are important for designing the adsorption system. The adsorption capacity of Pb 2+ by DSS/MIL-88A-Fe composite was evaluated using Langmuir (3), Freundlich (4), and Temkin (5) isothermal adsorption model to find the interaction between DSS/MIL-88A-Fe composite and Pb 2+ . The corresponding equations of isothermal adsorption models are as follows; (3) C e q e = 1 k L q L + C e q L ,  where q e (mg/g) is the equilibrium adsorption capacity of Pb 2+ in solution, q L (mg g −1 ) represents the maximum adsorption capacity, K L (L mg −1 ) is the Langmuir constant ascribed to the affinity of the binding sites between the adsorbent and the target substance. K F ((mg g −1 )/(mg g −1 ) 1/n ) and n ((mg (1−(1/n)) L (1/n) g −1 ) are Freundlich constants representing adsorption capacity and adsorption intensity, respectively. R is the gas constant (8.314 J/ mol⋅K), T is the temperature (K). Three models of experimental data from adsorption processes were applied to explain the Pb 2+ adsorption mechanism between the liquid and adsorbent phases. The fitting results of the adsorption of Pb 2+ are shown in Fig. 12. The parameters of the models are listed in Table 2.
The Langmuir model is normally associated with monolayer adsorption characteristics and the energy level of a homogeneous system, which has no following interaction between adsorbed species 151 . The Freundlich model is generally an empirical one related to heterogeneous systems and applied to multi-layer adsorption of the adsorbent 152 .
The Temkin model is commonly described by a uniform distribution of binding energies to explain adsorbate-adsorbent interactions on adsorption sites 153 .
The most favourable isothermal adsorption model was provided by the Freundlich model, as it yielded a higher R 2 value in contrast to the value of the correlation coefficient between the Langmuir and Temkin models. The adsorption behaviour of Pb 2+ by DSS/MIL-88A-Fe composite mainly occurred on heterogeneous pores or surfaces as the main adsorption sites, and multi-layer adsorption could exist. Effect of temperature and thermodynamic parameters. Temperature is one of the essential factors that influence the adsorption capacity of composite on absorbing Pb 2+ . The effect of temperature on the adsorption of Pb 2+ on DSS/MIL-88A-Fe composite was investigated at three temperatures at 298, 308, and 318 K. Obviously, on increasing the temperature, the adsorption capacity of the adsorbent on absorbing Pb 2+ increased. This showed that the adsorption process was endothermic.
To further investigate the thermodynamic features, the thermodynamic parameters such as Gibb's free energy change (ΔG°, kJ/mol), enthalpy change (ΔH°, kJ/mol), and entropy change (ΔS°, J/(mol·K)) were calculated using the following equations:   Fig. 13 and the thermodynamic parameters are provided in Table 3.
The negative values of ΔG° indicated the adsorption of Pb 2+ on DSS/MIL-88A-Fe composite was spontaneous. Moreover, with the elevated temperature, the absolute value of ΔG° increased, revealing that high temperature can promote the adsorption process.
The positive values of ΔH° suggest that the adsorption process was endothermic, in nature, whereas the positive value of entropy change (ΔS°) reveals the increase in randomness at the solid/solution interface during the adsorption of Pb 2+ . Therefore, the adsorption was an endothermic, and spontaneous process.  solution of pH 6 at room temperature were also studied. As shown in Fig. 8, the fast Pb 2+ adsorption process on DSS/MIL-88A-Fe composite in the first 5 min may be due to the presence of sufficient active adsorption sites available on the surface of the adsorbent. The adsorption was almost attained equilibrium within 30 min.
To study the adsorption kinetics and accurately interpret the adsorption behavior of Pb 2+ adsorption on DSS/ MIL-88A-Fe composite, four types of kinetic models, including the pseudo-first-order (9), pseudo-second-order (10), Elovich model (11), and particle diffusion (12) are expressed as follows 155,156 ; where q e and q t are the adsorption capacity at equilibrium and time (mg g −1 ), K 1 (min −1 ), and K 2 [g (mg min) −1 ] are adsorption rate constant of pseudo-first-order and pseudo-second-order kinetics, respectively. K i (mg/g min 0.5 ) and a are adsorption rate constant of intra-particle diffusion, and intra-particle diffusion constants that reflecting boundary layer effect, respectively. α[mg (g min) −1 ] and β (g mg −1 ) illustrate the initial constant adsorption and desorption constants, respectively [157][158][159] .
The adsorption process usually has various steps and out of which the slowest step controls the rate of the adsorption process.
The experimental data were fitted by using the above four adsorption kinetic models. According to the fitting results in Fig. 14a-d, and Table 4, it is evident that the R 2 value of the pseudo-second-order dynamics model is greater than that of the other three models (the correlation coefficient R 2 of the pseudo-second-order dynamic model is equal 0.998). This result indicates that the adsorption rate on the surface of the adsorbent is the ratedetermining step and the adsorbent surface corresponds to a heterogeneous system. (9) ln q e − q t = ln q e − K 1 .t,    www.nature.com/scientificreports/ Pb 2+ adsorption mechanism. According to the above analysis and characterization, a possible mechanism for Pb 2+ removal is suggested. To further understand the Pb 2+ adsorption process and the composite-heavy metal interaction, the zeta potential of the DSS and MIL-88(A)-Fe was measured at pH 6. On the basis of the obtained results, the surface charge of DDS was negative (− 40 mV) indicating the electrostatic attraction enhancing for more Pb 2+ adsorption. So, the mechanism of adsorption between DSS-heavy metals is due to the electrostatic attraction of unlike charges at pH 6. On the other hand, DSS has provided a high specific surface area and mesoporous channel microstructure in which many hydroxyl-functional groups were exposed on the surface of DSS materials as active sites for the adsorption of heavy metal ions. The adsorption has occurred mainly through electrostatic interactions between the surface hydroxyl group of DSS and the heavy metal ions.
Furthermore, the Pb 2+ adsorption mechanism in the solution via MIL-88(A)-Fe could readily happen by ion exchange protons on the surface of the adsorbent with Pb 2+ (This observation has been confirmed by comparison before and after Pb 2+ adsorption through FTIR and SEM analysis).
It could be understood that the Pb 2+ adsorption mechanism involved competitive ion exchange with MIL-88(A)-Fe and electrostatic interactions with the DSS of the composite. Also, the Pb 2+ adsorption mechanism could be occurred from binding to open metal sites on the MOF of composite (a pore-filling mechanism) or interacting with active sites on the surface of DSS containing hydroxyl-functional groups.
The  Fig. 15. The adsorption capacity of Pb 2+ by DSS/MIL-88(A)-Fe composite decreases slightly after 4 periods of regeneration to reuse the adsorbent (from 77.8% at the first cycle to 70.3% at the fourth cycle). The decrease in adsorption capacity during repeated use could be caused by the mass loss of the composite adsorbent in acid treatments. The results demonstrate good recycling capability in the cyclic adsorption process for Pb 2+ adsorption.
In order to further investigate the adsorption mechanism and the recycling capability of DSS/MIL-88(A)-Fe composite in the cyclic adsorption process for Pb 2+ adsorption, the FT-IR spectra of DSS/MIL-88(A)-Fe composite before (Fig. 16a), after adsorption of Pb 2+ (Fig. 16b), and also reused adsorbent after four catalytic runs (Fig. 16c) were analyzed.
As it is beheld from Fig. 16b, the characteristic peaks at 3420 and 1054 cm −1 attributed to vibrations of the DSS, shifted to 3407 and 1043 cm −1 and the intensity of the peaks weakened after the adsorption of Pb 2+ . Furthermore, the significant peaks at 1608 and 1398 cm −1 represented coordination between the carboxyl group and Fe 3+ , were red-shifted to 1568 and 1390 cm −1 , and significantly weakened (Supplementary Information).
Also, from the FT-IR spectrum of reused adsorbent after four catalytic runs can be concluded that were matched in all the characteristic absorption bands such as shapes, positions, and frequencies with the FT-IR spectra of the fresh catalyst (Fig. 16c).   Fig. 4f, the reused FE-SEM image of the DSS/MIL-88(A)-Fe composite asserts that not much change in the particle size or shape and morphology was observed after four catalytic runs.
A comparison of adsorption capacity and adsorption equilibrium time for Pb 2+ by DSS/MIL-88(A)-Fe nanocomposite with some different adsorbents is listed in Table 5. Obviously, in previously reported works, despite the short adsorption equilibrium time of Pb 2+ , adsorbents demonstrated poor adsorption capacity compared to this work ( Table 5, entries 1-6). Also, modified biochar showed a long adsorption time and relatively low adsorption capacity (Table 5, entry 7). In this study, the DSS/MIL-88(A)-Fe nanocomposite as an effective adsorbent of Pb 2+ from an aqueous solution has high adsorption performance as well as a short adsorption time.

Conclusions
In summary, we synthesized a double-shelled periodic mesoporous organosilica nanospheres/MIL-88(A)-Fe nanocomposite in a conventional manner and comprehensively characterized through various techniques, including FTIR, XRD, BET, TEM, FE-SEM, EDX, and EDX-mapping analysis. Thanks to unique structure and remarkable properties of DSS/MIL-88A-Fe composite (such as an average size of 280 nm and 1.1 μm long attributed to the DSS and MOF, respectively, microporous structure and relatively large specific surface area (312.87 m 2 /g), which resulted from the coexistence of PMO and MOF, the composite described above exhibited excellent performance in the separation of Pb 2+ from water with a maximum adsorption capacity of 230 mg/g with an effective adsorption rate of around 90 min. More importantly, one of the main advantages of this unprecedented composite is its cycling stability. To this end, the reusability of the DSS/MIL-88(A)-Fe was investigate, and the obtained results demonstrated that this adsorbent was preserved after 4 times regeneration, which illustrated its favorable performance in the removal of lead metal pollutants.

Data availability
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.